1. Field of the Invention
The present invention relates to novel compositions which can be formed into metal oxide films having high refractive indices. The compositions are useful for forming solid-state devices such as flat panel displays, optical sensors, integrated optical circuits, and light-emitting diodes.
2. Description of the Prior Art
The performance of many solid-state devices including flat panel displays, optical sensors, integrated optical (photonic) circuits, and light-emitting diodes (LEDs) can be improved by applying a transparent, high refractive index coating onto the light-emitting or light-sensing portion of the device structure. Organic polymer coatings offer easy, low-temperature application and robust mechanical properties, including good surface adhesion, when used on such devices. However, few organic polymers have refractive indices greater than 1.65 at visible wavelengths, and fewer still have indices of 1.70 or greater. Those polymers that do have higher indices generally contain a high concentration of large polarizable atoms such as bromine, iodine, or sulfur, which limits their thermal and chemical stability.
On the other hand, certain metal oxides, most notably those of titanium and zirconium, possess excellent optical clarity when applied as thin films and exhibit refractive indices of 2.0 or more at visible wavelengths. They, unfortunately, must be deposited by expensive and inefficient methods such as evaporation or sputtering, and then can only be applied as thin films (less than 1 μm in thickness) whereas device makers are often seeking films of several microns to several tens of microns in thickness. Moreover, the deposited metal oxide coatings are brittle and may not adhere well to device surfaces without high-temperature annealing, which may degrade device operation.
The well known sol-gel coating method has been used to deposit high index metal oxide coatings from solution. However, the coatings tend to be brittle and subject to cracking and require long, complicated curing schedules. Sol-gel coating solutions also have limited pot life, making the method difficult to practice on a commercial scale. More recently, the sol-gel method has been combined with conventional polymer chemistry to prepare inorganic-organic hybrid coatings in which the metal oxide phase, formed by in situ hydrolysis and condensation of a metal alkoxide, is chemically bonded to an organic polymer phase to obtain materials with greater toughness and durability. However, they are still prone to the pot life problems associated with sol-gel compositions and lend themselves best to silicon dioxide incorporation, which does not promote a high refractive index.
Inorganic-organic hybrid coatings have also been prepared by dispersing nanosized (1 to 50 nm in diameter) metal oxide particles in a polymer vehicle to produce transparent film compositions. However, the refractive indices of these compositions are largely restricted to the range of 1.55 to 1.70 unless very high metal oxide loadings (80%) are utilized. Moreover, preparation of the coatings requires many steps, including particle synthesis and purification, surface treatment, and dispersion, often under a non-ambient environment.
Therefore, in light of the shortcomings of the prior art, a need exists for coatings that have a refractive index greater than about 1.7, and preferably greater than about 1.75, at visible and near infrared wavelengths (from about 400-1700 nm), and that provide high optical clarity along with easy and reliable preparation, long shelf life, and good mechanical stability at film thicknesses of greater than 1 micron.